In digital transmission systems such as those described in the parent application, and in particular in packet transmission systems such as those operating in asynchronous transfer mode (ATM), there may be slight differences in timing between a transmitting station and a remote receiving station, which differences can result in a perceptible degradation in the received data. In order to faithfully re-establish the transmitted timing at a receiving station to which data has been directed, a time-stamp is sometimes included with the data, such time-stamp being an indication of the frequency difference between the incoming signal and a local clock derived from a central system clock which the transmitting and receiving stations share. Such time-stamps may, for example, be generated for systems operating in ATM in accordance with Bellcore specification TA-NWT-00113, with the resulting time-stamp sometimes being referred to as the synchronous residual time-stamp (SRTS).
While systems for putting a time-stamp on each data packet being routed through a transmission system are known, simple and efficient ways for retrieving and utilizing the time-stamp to synchronize the receiving station are less well known. Further, since in ATM and other packet-based data transmission techniques the data is received asynchronously, it is desirable that a buffer, for example a FIFO (i.e., first-in first-out) buffer, be provided between the incoming line and the remainder of the receiving station to smooth the received data. Optimally, this buffer is maintained at a selected fill level, for example approximately half full, so as to be able to continuously output at a lower speed the high speed asynchronously received input packets without loss of data due to buffer underflow or overflow.
One potential problem is that time-stamps may be missing for some reason, causing the buffer to "walk" (i.e., the buffer will either become overfilled or will empty preventing the smooth flow of data into the receiving station). This can cause aberrations in the received data and could result in received data being lost. A time-stamp may be missing from received data either because the time-stamp for some reason did not get added to the data at the transmitting station, the time-stamp somehow got lost in the transmission, the time-stamp was missed at the receiving station, or one or more data packets are not received at the receiving station. In any of these events, it is desirable that appropriate remedial action be taken at the receiving station to maintain an acceptable fill level in the buffer and to maintain an acceptable output rate for data from the buffer into the remainder of the receiving station.